PROJECT SUMMARY Accurate translation of the genetic information from messenger RNA to protein depends on multiple quality control mechanisms, which collectively maintain the average levels of translational errors at 10-4 for amino acid misincorporation (missense errors) and 10-2 for stop codon readthrough. However, increasing evidence shows that such translational fidelity is not fixed, but is rather affected by various environmental cues and genetic factors. Currently, we are only beginning to understand the regulatory networks leading to the fluctuation of translational fidelity during environmental changes and the resulting physiological responses. Translational errors can lead to reduced fitness, such as growth defects in bacteria and neurodegeneration in mammals, but may also benefit cells under certain stress conditions. Recently, we have shown that translational error rates vary from cell to cell in a genetically-identical bacterial population, raising the interesting question as to how fluctuation of translational errors affects the behavior of individual cells. In my laboratory, we are interested in developing and applying new technologies to understand the regulation and physiological roles of translational fidelity at both the population and single-cell levels. We are using our recently developed high-throughput reporter system to screen for conditions that alter translational fidelity, and have already identified novel environmental and genetic factors that are critical for this process. Next, we will determine the underlying mechanisms and expand our screens. Another research direction is to study how translational fidelity affects bacteria-host interactions, which is poorly understood. Our recent work reveals that either decreasing or increasing translational fidelity impairs expression of virulence genes and motility in Salmonella, suggesting that an optimal level of translational errors benefit bacteria during host interactions. We will further determine the regulatory networks using population and single- cell approaches. These studies will provide important insights into the roles of translational fidelity in environmental adaption and the regulatory mechanisms.